The invention relates to a process for producing xerogel of silicic acid with high volume of pores applicable especially for high performance liquid chromatography.
Employment of polymeric silicic acid/silica gel as column packing in liquid chromatography utilizes mainly high mechanical strength and stability of this material which, with spherical particles of similar size, and with sufficient volume of pores of suitable size, ensures high homogeneity and performance of the packing. Constant volume and shape of silica gel particles also makes it possible to change the eluent in the column without causing any macroscopic alterations of chromatographic bed volume and formation of heterogeneities.
The specific volume of pores is one of the important criteria for suitability of these materials. The higher the volume of pores, the higher the selectivity in gel permeation chromatography and the sample capacity in sorption liquid chromatography which can be reached at separation. This means that an equal separation effect can be achieved with a smaller column; in other words within a shorter time and with smaller eluent volume. At present, the volume of pores of the commercially available silica gel materials is mostly 0.5 to 0.8 cm.sup.3 g.sup.-1 and up to 1.5 cm.sup.3 g.sup.-1 at the most.
There are known in the prior art the processes for preparation of silica gels with higher volume of pores. For example, U.S. Pat. No. 3,526,603 discloses treating the silica gel with hot ammonia. Mineral acid or any medium strength acid is added for neutralizing the ammonia or for removing the salt excess. Such prepared silica gels have only medium pore size up to 1.25 ml g.sup.-1.
Unger and Scharf (J. Coll. Interface Sci. 55, (1976), 371), on the other hand, prepared on laboratory scale silica gels with specific volumes up to 4 ml g.sup.-1, but they used an expensive raw material--polyethoxysilane--and pretentious, hardly reproducible in technological scale, technique.
The main problem in obtaining aerogels with high volume of pores, using a cheap and readily available raw material--alkali metal silicates--as a source of silicic acid, is connected with the insufficient mechanical strength of silica hydrogel which is formed by polymerization of silicic acid in the first step of the process. Said hydrogel contains large amount water located in pores, which must be removed. However, water, because of both its high dipole moment and surface tension, causes extensive shrinkage of soft silica matrix during conventional drying and, consequently, substantial decrease of pore volume of resulting aerogel.
This disadvantage is partially overcome in different manner. Thus, for example, in U.S. Pat. No. 3,652,216 the hydrogel is washed and dried by long-time azeotropic distillation with water-immiscible solvent. U.S. Pat. No. 3,652,214 removes all water present in the hydrogen through vacuum sublimation after freezing it to such temperature that all water in pores is in a frozen state or, replacing it with organic solvent prior drying. Also in U.S. Pat. No. 3,819,811 which discloses a process of silica gel preparation by critically controlled steps of precipitation by neutralizing an alkaline silicate solutions with diluted mineral acids, ageing the hydrogel slurry at pH ranging from 3 to 8, washing with solutions of salts which displace acid-base salts, e.g. with aluminium chloride, ammonium chloride, or nitrate, or with diluted water solutions of mineral acids in which pH values does not fall below 3. The water from the hydrogel is displaced again either by washing with organic water-miscible liquids or by freeze drying technique or by azeotropic distillation.
According to U.S. Pat. No. 4,206,297 the silica hydrogel which is formed by reacting an organosilicate ester with diluted mineral acid--alcohol containing solutions at strictly controlled conditions (time, temperature, pH value) is washed with water-miscible solvents and dryied. Similarly, in U.S. Pat. No. 4,169,926 is disclosed contacting the silica-containing hydrogels with organic oxygen-containing water miscible solvents alone, or in a mixture with a normally liquid hydrocarbons or in combination with said hydrocarbons plus a minor amount of surfactants or their mixtures, followed by drying to get improved pore volumes of said silica-containing gels.
U.S. Pat. No. 4,089,932 relates to process of production of spherical silica gel comprising emulsification and acidification in a mixture of polar and non-polar organic solvent, washing with alcohols or ketones and drying and heating.
Another way to overcome the shrinking effect during drying is described in U.S. Pat. No. 3,977,993, which involves washing silica hydrogel with diluted solution of acid or ammonium salt to free it from alkali metal ions, followed by displacing water from the hydrogel by solvents miscible with water and heating to a temperature exceeding the critical temperature of the said organic solvent in an autoclave.
Some other processes known in the prior art disclose influencing the soft silica hydrogel matrix as, for example, in U.S. Pat. No. 3,959,174 where the silica gel is prepared in the presence of desolubizing agent as ammonia, sodium sulphate, and other inorganic salts, as well as some organic substances. Washing in acetone is used for dewatering. Also in U.S. Pat. No. 4,104,363 the strength of hydrogel is increased by hydrothermal treatment or by heating in neutral organic solvents.
The above procedure lead to the silicagels with increased volume of pores--till 3.0 cm.sup.3 /g at the most which is still relatively far from the values needed for special chromatographic applications.